The present invention relates to a novel method of forming a semiconductor thin film and a method of fabricating a solar cell using the above method.
Conventional techniques to form a semiconductor thin film consisting of group IV element, i.e., silicon or germanium include vapor phase growing processes such as evaporation, sputtering, ion plating and CVD. These techniques however essentially require an use of a highly sealed reactor because they use a reduced pressure or hydrogen atmosphere upon film formation. Therefore, it is difficult to form a uniform semiconductor thin film on a large area substrate. This limits fabrication of products including a semiconductor thin film such as a solar cell, for example, having a larger area of an order of magnitude compared with any conventional product.
One technique to form a large area semiconductor thin film includes a use of liquid coating method. Japanese Laid-Open Patent Application Publication No. 4-119996 discloses a method of forming a silicon thin film by coating a silicon source compound such as octasilacubane on a substrate and thermally decomposing the silicon source compound. The method does not require any large-scaled expensive fabrication equipment such as a vacuum chamber and is advantageous in view of easily forming a silicon thin film on a large area substrate. However, such a compound as octasilacubane is disadvantageous in that it is unstable to oxygen and is easily oxidized and deteriorated in the air. Also such a compound have poor solubility into a solvent and is difficult to form a uniform thin film.
Likewise, conventional methods to form a compound semiconductor thin film include processes such as evaporation, sputtering, MOCVD and molecular beam epitaxy. These methods require a highly airtight reactor in a film formation process in order to prevent oxygen in the atmosphere from mixing into the reactor, so that they are inadequate to form a uniform compound semiconductor thin film on a large area substrate. In MOCVD (Metal Organic Chemical Vapor Deposition), for example, source materials of compound semiconductor constituents are fed to the reactor as gases, to form a compound semiconductor thin film by a solid deposition reaction from gas phase. Accordingly, the area of a substrate on which deposition is performed is limited by the size of the reactior.
In order to improve the problems, there is provided a technique in Japanese Laid-Open Patent Application Publication No. 5-503319 in which a precursor consisting of complex compound containing III group and V group elements is used to deposit a III-V group compound semiconductor on a substrate. However, there are only a few molecular species matching desired conditions of deposition reaction and that a ratio between a III group element and a V group element can not be arbitralily controlled. The method is therefore not used practically.
Further, when a solar cell is fabricated using a silicon thin film, it is necessary to form a silicon thin film doped with p- or n-type impurity. In the prior art, however, a silicon thin film is formed by thermal decomposition, and then impurities are doped by thermal diffusion or ion implantation so that a large-scaled apparatus is required to cause complication of fabrication processes.